Pei-Yao Guo-Wang‡
a,
Jian-Xun Ding‡b,
Wei Guoa,
Hui-Yong Wua,
Jun-Chao Wei*a,
Yan-Feng Daia and
Feng-Jie Denga
aCollege of Chemistry, Nanchang University, Nanchang 330031, P.R.China. E-mail: weijunchao@ncu.edu.cn
bKey Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
First published on 5th January 2016
A critical challenge in the preparation of reinforced polymer nanocomposites is to prevent the aggregation of nanofillers. In this work, a novel poly(γ-benzyl-L-glutamate) (PBLG)-modified silicon dioxide@graphene oxide nanofiller (SiO2@GO-g-PBLG) was prepared via a continuous electrostatic complex and ring-opening polymerization (ROP) method. The grafted PBLG prevented the aggregation of SiO2 nanoparticles and GO sheets, realized the colloid stability of SiO2@GO-g-PBLG in organic solvents, and thus increased its phase interaction in a polymer matrix. The hybrid nanofiller greatly enhanced the mechanical properties of poly(L-lactide) (PLLA). As a typical feature, the tensile strength of PLLA nanocomposites with only 5 wt% of SiO2@GO-g-PBLG was 88.9 MPa, about 45% higher than that of pure PLLA. In addition, the hybrid nanofillers also have some positive effects on improving the thermal stability of PLLA. Therefore, SiO2@GO-g-PBLG was a promising hybrid nanofiller to reinforce polymers such as PLLA.
One common approach to hinder the aggregation of graphene sheets is to prepare the inorganic nanoparticle–graphene hybrids.8,9 When the inorganic nanoparticles are anchored on the surface of graphene, it can reduce the π–π interaction; on the other hand, the aggregation of inorganic nanoparticles can also be prevented. Up to now, many inorganic nanoparticle–graphene hybrids have been prepared.9 More interestingly, much better synergistic reinforcing effect toward polymer nanocomposites may appear and realize, when different types of nanofillers are simultaneously added into polymer matrix. Many hybrid nanofillers consisting of two or more components, such as halloysite–graphene,10 carbon nanotube–graphene,11 and carbon nanotube–halloysite,12 have been prepared and used to reinforce various polymer matrix, which have shown more satisfying results than their single component. However, these hybrid nanofillers are all inorganic components, there still lack a bridge to connect the inorganic nanofillers and the polymer matrix.
As mentioned above, owing to the poor dispersion or non-regular arrangement of nanofillers, the polymer nanocomposites always have disappointing mechanical properties and fall far below their theoretical values.13,14 In order to further improve the mechanical performances of polymer nanocomposites, an effective method is to graft polymer chains on the surface of nanofillers and thus enhance the phase compatibility between nanofillers and polymer matrix.
Poly(L-lactide) (PLLA) is an important biodegradable polymer, which has been widely used in various biomedical fields or as a disposable package. However, the low mechanical properties have limited its final application; so many PLLA nanocomposites have been designed and prepared.15 For example, Chen's group prepared the PLLA-modified hydroxyapatite (HA-g-PLLA) and found that the tensile strength of HA-g-PLLA/PLLA nanocomposites can be improved to 75 MPa, which indicated much potential applications in bone tissue engineering.16,17 Recently, the graphene/PLLA nanocomposites have also been reported, and some researchers also use the polymer-modified graphene to blend with PLLA.18,19 For example, PLLA-grafted graphene oxide (GO-g-PLLA) was used to reinforce the PLLA matrix, and the tensile strength of GO-g-PLLA/PLLA could be about 75 MPa.20 Up to now, it is still a challenge to acquire PLLA nanocomposites with sufficient mechanical strength.
In order to hinder the aggregation of nanofillers, enhance their phase compatibility with polymer matrix, and also obtain high mechanical strength of PLLA nanocomposites, a novel ternary hybrid nanofiller, i.e., SiO2@GO-g-PBLG, was prepared. Typically, as shown in Fig. 1, silicon dioxide@graphene oxide (SiO2@GO) was firstly prepared. And then, the biodegradable poly(γ-benzyl-L-glutamate) (PBLG) was grafted onto the surface of SiO2@GO, forming a ternary organic–inorganic hybrid, that is, SiO2@GO-g-PBLG. The newly prepared hybrid can prevent the aggregation of SiO2 nanoparticle and graphene sheet, and realize the homogeneous dispersion in polymer matrix. More importantly, the PBLG chains can bridge SiO2@GO and PLLA matrix, and thus the PLLA nanocomposites with excellent strength were obtained.
γ-Benzyl-L-glutamate N-carboxyanhydride (BLG NCA) was synthesized according to the previously published method.23 Tetraethylorthosilicate (TEOS) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, P. R. China). 3-Aminopropyltriethoxysilane (APTES) was obtained from Sigma-Aldrich (Shanghai, P. R. China). 1,6-Diaminohexane was purchased from Tokyo Chemical Industry Co., Ltd. (Shanghai, P. R. China). PLLA was donated by Zhejiang Hisun Biomaterials Co., Ltd. (Taizhou, P. R. China). All the chemicals were used without further purification.
Secondly, SiO2@GO was prepared by a simple electrostatic complex technique. Briefly, SiO2–NH2 (300.0 mg) and GO (700.0 mg) were dispersed in deionized water (300.0 mL) by ultrasonic and stirred at room temperature for 24 h. And then, the resultant product was obtained by centrifugation and washed with deionized water several times to remove the residual GO.
To synthesize SiO2@GO-g-PBLG, SiO2@GO-NH2 (250.0 mg) and BLG NCA (500.0 mg) were added in a dry flask under the protection of nitrogen (N2), and then anhydrous N,N-dimethylformamide (DMF) (30.0 mL) was injected into the flask by syringe. The mixture was dispersed with ultrasound for 30 min, and then stirred at 35 °C for 48 h. The final product, that is, SiO2@GO-g-PBLG, was separated by centrifugation, washed with DMF three times, and dried under vacuum.
The TGA curves of GO, SiO2, SiO2@GO-NH2, and SiO2@GO-g-PBLG were shown in Fig. 3. The weight losses of different samples under 150 °C should be attributed to the volatilization of adsorbed water. As for GO and SiO2, when the samples were heated to above 150 °C, the surface functional groups would decompose, and thus there was an evident weight loss for both GO and SiO2@GO. Comparing the TGA curve of SiO2@GO-NH2 with the TGA of SiO2@GO-g-PBLG, the effective weight losses of SiO2@GO-NH2 and SiO2@GO-g-PBLG in the temperature range of 150 to 800 °C were 15.70 and 25.9 wt%, respectively, and these different values were derived from the decomposition of PBLG chain.
The morphologies of SiO2@GO and SiO2@GO-g-PBLG were observed by both SEM and TEM. As shown in the SEM micrographs of Fig. 4, the spherical structure of SiO2 in SiO2@GO was clear, and GO sheet was observed on the structure of SiO2@GO nanoparticle. In addition, the size of GO sheet was much bigger than that of SiO2, so it is possible that GO sheet could connect several SiO2 nanoparticles together. After the polymerization of BLG NCA, PBLG chains were grafted on the surface of SiO2@GO, resulting in the formation of polymer layer on the surface of SiO2@GO. The TEM images of SiO2@GO and SiO2@GO-g-PBLG were also represented in Fig. 2. It can well demonstrated that GO was coated on the surface of SiO2, meanwhile it also showed that GO sheet could connect SiO2 nanoparticles together. As for the structure of SiO2@GO-g-PBLG, the hydrophobic PBLG chains existed on the surface of SiO2@GO, and thus altered the surface properties of SiO2@GO. More importantly, the PBLG chains might be of vital importance when the hybrid was used to reinforce polymers, because this might increase the interactive area and phase compatibility between nanofiller and polymer matrix.
As shown in Fig. 5, SiO2@GO and SiO2@GO-g-PBLG were added into a two phase solvents, i.e., the upper and lower layers were water and chloroform, respectively. After ultrasonic dispersion, SiO2@GO dispersed homogeneously in the water phase, while SiO2@GO-g-PBLG existed in chloroform and maintained its colloid stability for seven days. This result well demonstrated that the surface wettability of SiO2@GO-g-PBLG was hydrophobic, which was much different with that of SiO2@GO. So it may be very useful when SiO2@GO-g-PBLG was blended with organic polymers. The PBLG chains might enhance the phase interaction between the nanofiller and polymer matrix, and thus obtain better mechanical properties.
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Fig. 6 Tensile stress–strain curves of PLLA and SiO2@GO-g-PBLG/PLLA nanocomposites with different SiO2@GO-g-PBLG contents. |
Samples | Tensile strength (MPa) | Modulus (GPa) | Elongation at break (%) |
---|---|---|---|
a PLLA-SG5 is a control composite sample with 5 wt% of SiO2@GO. | |||
PLLA | 64.0 ± 3.0 | 2.4 ± 0.3 | 4.3 ± 0.6 |
PLLA0.5 | 75.2 ± 2.8 | 2.9 ± 0.3 | 3.5 ± 0.5 |
PLLA1 | 79.0 ± 1.8 | 3.2 ± 0.10 | 3.4 ± 0.1 |
PLLA2 | 83.7 ± 4.5 | 3.3 ± 0.2 | 3.4 ± 0.2 |
PLLA5 | 88.8 ± 3.2 | 3.5 ± 0.3 | 3.6 ± 0.2 |
PLLA-SG5a | 70.5 ± 4.3 | 3.1 ± 0.4 | 2.8 ± 0.1 |
The morphologies of the tensile fracture surfaces for both PLLA and different PLLA nanocomposites were shown in Fig. 7. The fracture surface of PLLA was smooth, while the surfaces for PLLA nanocomposites were a little coarser than that of PLLA. Furthermore, some fibrous structure appeared on the fracture surfaces of PLLA nanocomposites, which might result from the reinforcement of SiO2@GO-g-PBLG. To more clearly observe the surface morphology, a high magnification microimage of the fracture surface of PLLA5 was shown in Fig. 7. It showed clearly that the hybrid nanofillers dispersed homogeneously in the polymer matrix without the aggregation of nanoparticle. The phenomenon might be because that the phase compatibility between the SiO2@GO-g-PBLG nanofiller and PLLA matrix was much better. Furthermore, as denoted by the arrows in Fig. 7, the interface between nanoparticle and polymer was obscure, and some wrinkles were found between the nanoparticle and polymer. The results implied that the phase interaction was much stronger, and this point might be an explanation for the improved mechanical properties as shown in Table 1.
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Fig. 7 Fracture surface morphologies of PLLA and its nanocomposites with different SiO2@GO-g-PBLG contents. |
The thermal properties of SiO2@GO-g-PBLG/PLLA nanocomposites were also investigated by TGA and DSC. As shown in Fig. 8 and S1, ESI,† lower SiO2@GO-g-PBLG contents (e.g., 0.5 and 1 wt%) had little effect on the thermal stability of PLLA. When the content of nanofiller was more than 2 wt%, the thermal stability of PLLA nanocomposite was better than that of pure PLLA. On the other hand, the hybrid nanofiller showed quite slight effect on the melting temperature (Tm) and crystallization temperature (Tc). As shown in Fig. 9, the Tm of PLLA was 177.8 °C, while those of the PLLA nanocomposites were all in the range of 177.2–177.7 °C. The Tc of PLLA and its nanocomposites exhibited no significant difference. All the Tc values were in the range of 106.6–107.5 °C, and the Tc of PLLA5 sample was only 0.5 °C higher than that of PLLA. Furthermore, the SiO2@GO-g-PBLG nanofillers might greatly enhance the crystallization speed of PLLA, when isothermally crystallized in a higher temperature. These results will be discussed in our future work.
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Fig. 8 TGA decomposition graphs of pure PLLA and its nanocomposites with different SiO2@GO-g-PBLG contents. |
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Fig. 9 DSC thermal graphs of PLLA and its nanocomposites with different SiO2@GO-g-PBLG contents during the heating (A) and cooling process (B). |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27104e |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2016 |